Research by a Lawrence Livermore National Laboratory (LLNL) physicist and a
host of collaborators is shedding new light on one of the major challenges
to realizing the promise and potential of quantum computing — error
correction.
In a new paper published in Nature and co-authored by LLNL physicist
Jonathan DuBois, scientists examined quantum computing stability,
particularly what causes errors and how quantum circuits react to them. This
must be understood in order to build a functioning quantum system. Other
co-authors included researchers at the University of Wisconsin-Madison,
Fermi National Accelerator Laboratory, Google, Stanford University and
international universities.
In experiments performed at UW-Madison, the research team characterized a
quantum testbed device, finding that fluctuations in the electrical charge
of multiple quantum bits, or “qubits” — the basic unit of a quantum computer
— can be highly correlated, as opposed to completely random and independent.
When a disruptive event occurs, such as a burst of energy coming from
outside the system, it can affect every qubit in the vicinity of the event
simultaneously, resulting in correlated errors that can span the entire
system, the researchers found. Additionally, the team linked tiny
error-causing perturbations in the qubits’ charge state to the absorption of
cosmic rays, a finding that already is impacting how quantum computers are
designed.
“For the most part, schemes designed to correct errors in quantum computers
assume that the errors across qubits are uncorrelated — they’re random.
Correlated errors are very difficult to correct,” said co-author DuBois, who
heads LLNL’s Quantum Coherent Device Physics (QCDP) Group. “Essentially,
what this paper is showing is that if a high-energy cosmic ray hits the
device somewhere, it has the potential to affect everything in the device at
once. Unless you can prevent that from happening you can’t perform error
correction efficiently, and you’ll never be able to build a working system
without that.”
Unlike bits found in classical computers, which can exist only in binary
states — zeroes or ones — the qubits that make up a quantum computer can
exist in superpositions. For a few hundred microseconds, data in a qubit can
be either a one or zero before being projected into a classical binary
state. Whereas bits are only susceptible to one type of error, under their
temporary excited charge state, the delicate qubits are susceptible to two
types of error, stemming from changes that can occur in the environment.
Charged impulses, even minute ones like those from cosmic rays absorbed by
the system, can create a blast of (relatively) high-energy electrons that
can heat up the quantum device’s substrate just long enough to disrupt the
qubits and disturb their quantum states, the researchers found. When a
particle impact occurs, it produces a wake of electrons in the device. These
charged particles zoom through the materials in the device, scattering off
atoms and producing high-energy vibrations and heat. This alters the
electric field as well as the thermal and vibrational environment around the
qubits, resulting in errors, DuBois explained.
“We’ve always known this was possible and a potential effect — one of many
that can influence the behavior of a qubit,” DuBois added. “We even joked
when we saw bad performance that maybe it’s because of cosmic rays. The
significance of this research is that, given that sort of architecture, it
puts some quantitative bounds on what you can expect in terms of performance
for current device designs in the presence of environmental radiation.”
To view the disruptions, researchers sent radio frequency signals into a
four qubit system and, by measuring their excitation spectrum and performing
spectroscopy on them, were able to see the qubits “flip” from one quantum
state to another, observing that they all shift in energy at the same time,
in response to changes in the charge environment.
“If our model about particle impacts is correct, then we would expect that
most of the energy is converted into vibrations in the chip that propagate
over long distances,” said UW-Madison graduate student Chris Wilen, the
paper’s lead author. “As the energy spreads, the disturbance would lead to
qubit flips that are correlated across the entire chip.”
Using the method, researchers also examined the lifetimes of qubits — the
length of time that qubits can remain in their superposition of both one and
zero — and correlated changes in the charge state with a reduction in
lifetime of all the qubits in the system.
The team concluded that quantum error correction will require development of
mitigation strategies to protect quantum systems from correlated errors due
to cosmic rays and other particle impacts.
“I think people have been approaching the problem of error correction in an
overly optimistic way, blindly making the assumption that errors are not
correlated,” said UW-Madison physics professor Robert McDermott, senior
author on the study. “Our experiments show absolutely that errors are
correlated, but as we identify problems and develop a deep physical
understanding, we’re going to find ways to work around them.”
Though long theorized, DuBois said the team’s findings had never been
experimentally proven in a multi-qubit device before. The results will
likely impact future quantum system architecture, such as putting quantum
computers behind lead shielding or underground, introducing heatsinks or
dampers to quickly absorb energy and isolate qubits, and alter the types of
materials used in quantum systems.
LLNL currently has a quantum computing testbed system, designed and built
with funding from a Laboratory Directed Research and Development (LDRD)
Strategic Initiative that began in 2016. It is being developed with
continued support by the National Nuclear Security Administration’s Advanced
Simulation & Computing program and its Beyond Moore’s Law project.
In related follow-on work, DuBois and his team in the QCDP group are
studying a quantum device that is significantly less sensitive to the charge
environment. At the extreme cold temperatures required by quantum computers
(systems are kept at temperatures colder than outer space), DuBois said
researchers observe that thermal and coherent energy transport is
qualitatively different from room temperature. For example, instead of
diffusing, thermal energy can bounce around in the system like sound waves.
DuBois said he and his team are focused on understanding the dynamics of the
“microscopic explosion” that occurs inside quantum computing devices when
they interact with high energy particles and developing ways to absorb the
energy before it can disrupt the delicate quantum states stored in the
device.
“There are potentially ways to design the system so it’s as insensitive as
possible to these kinds of events, and in order to do that you need to have
a really good understanding of how it heats up, how it cools down and what
exactly is happening through the whole process when exposed to background
radiation,” DuBois said. “The physics of what’s going on is quite
interesting. It’s a frontier, even aside from the quantum applications,
because of the oddities of how energy is transported at those low
temperatures. It makes it a physics challenge.”
DuBois has been working with the paper’s principal investigator McDermott
(UW-Madison) and his group to develop ways to use qubits as detectors to
measure charge bias, the method the team used in the paper to conduct their
experiments.
Reference:
Correlated charge noise and relaxation errors in superconducting qubits by
C. D. Wilen, S. Abdullah, N. A. Kurinsky, C. Stanford, L. Cardani, G.
D’Imperio, C. Tomei, L. Faoro, L. B. Ioffe, C. H. Liu, A. Opremcak, B. G.
Christensen, J. L. DuBois and R. McDermott, 16 June 2021, Nature.
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Physics